The present invention relates to a method of producing propeller blades and to blades obtained by such method.
As is well known, propellers are devices which comprise one or more blades, fixed to a rotatable shaft, and designed to produce a relative movement between the plane of rotation of the propeller and a flowable material surrounding the propeller, the relative speed of the fluid streams in comparison to the plane of rotation of the propeller remaining, over the entire surface swept by the propeller, as parallel as possible to the rotational axis of the propeller.
Propellers may be used either as a driving means, when working in a relatively unlimited fluid medium (marine or aeronautic propellers) or for circulating a flowable material in a closed circuit (ventilators, mixers etc.).
With, for example, a propeller for a mixer, the main aims are basically:
(A) to cause the passage into the zone of the propeller of the miscible or non-miscible phases contained in the mixer-container and to cause them to circulate therein in order to attain the desired homogeneity of the mixture. PA1 (B) to ensure that, at all points of the mixture in circulation, speeds of suitable rates and direction are maintained to prevent separation of the mixture. PA1 .omega. is the angular rotational speed of the propeller PA1 .omega.R is the tangential speed of the blade for a radius R under consideration PA1 l is the length of the chord of the cylindrical profile, PA1 .beta. is the setting angle, i.e. the angle formed by the chord with a plane perpendicular to the axis of rotation, PA1 V is the absolute speed (assumed to be parallel to the axis of rotation) of the incident fluid medium, PA1 W is the relative speed of the fluid medium in relation to the leading edge of the blade (W=V-.omega.R), PA1 V' is the absolute speed of exit of the fluid medium, PA1 i is the angle of incidence of W relative to the chord l, PA1 .alpha. is the angle of W relative to the speed .omega.R(tg.alpha.=tg(.beta.-i)=(V/.omega.R) PA1 .zeta. is the rise of camber of the profile.
Moreover, in order to prevent the mixture from rotating as a body within the container, the containers are usually provided with fixed anti-rotation devices so that rotation of the mixture about the axis of the propeller is avoided and, the streams of mixture reaching the leading edge of the propeller blades have a displacement which is substantially parallel to the axis of rotation of the propeller.
In the present state of the art, the classical method of developing a propeller blade consists of successively studying different cross sections of the blade at pitch circles concentric with the axis of rotation e.g. at radii R.sub.1, R.sub.2 . . . R.sub.n between the external radius Rex of the blade and the internal radius R.sub.i and which radii Rex and R.sub.1 defining between them the effective radial length of the propeller. It should be noted that a so called "cylindrical profile" for a fixed radius R, comprises the cross section of the blade at radius R, from the axis of rotation, and that a "development of the cylindrical profile" comprises a flat development of this arcuate cross section. If the thickness of a cylindrical profile is slight relative to its chord, it is possible to consider the profile as a line without thickness (slim profile).
In this specification, the following terms have been adopted:
After having chosen a general form for the propeller, the establishment of the cylindrical profile for a determined radius R is effected by using the above noted classical method which allows calculation by successive approximations of the chord l, the setting angle .beta., the angle of incidence i (this angle should be sufficiently near its optimum value for which the ratio (drag/lift is minimal).
For an accurately determined cylindrical profile, the angle i is small in relation to .beta. and, at first approximation, the theoretical value of the axial component of the speed V=.omega.R tg(.beta.-i) is in the region of .omega.Rtg.beta.. The pitch of the blade for the rotation radius R is 2.pi.Rtg .beta..
In order to define the respective cylindrical profiles corresponding to different blade radii, relations are sometimes used which join the value of the parameter .beta. to that of the corresponding radius R. In particular, in the case of propellers known as "constant pitch", such as the classical "marine" propellers, the product R.multidot.tg .beta. is maintained constant over the entire length of the blade.
Having established the cylindrical profiles for different radii (for example R = R.sub.ex, R = 0.75 R.sub.ex, R = 0.5 R.sub.ex, R = 0.3 R.sub.ex), the surface of the blade is the surface enclosing these different profiles. One can choose from an infinite variety of relative positions of a cylindrical profile corresponding to a certain radius in relation to that corresponding to another radius by making them slide relatively parallel to the axis of rotation or to turn about this axis. The choice of relative positions is generally made by successive tests in terms of other criteria which may be of construction, surface development, aesthetical appearance or the like.
The classical method described above does not take into account the fact that, in practice, the courses of the fluid streams while crossing the propeller deviate from the theoretical courses defined by the cylindrical profiles of the blades, particularly because the drag effect of the blade induces a tangential component of the speed of flux which increases from the leading edge to the trailing edge of the blade. Designating U as the value of this tangential component of the speed at a certain point of the trailing edge of the blade and V as the value of the axial component at this point, the ratio U/V increases (all other things being equal) as the angle .alpha. increases, as the radius R decreases, as the coefficient of loss of charge of the hydraulic circuit increases (therefore as the cinematic viscosity .mu. of the medium rises above a specific threshold).
For a weak pitch propeller turning in an unlimited incompressible medium the centrifugal effect produced by the tangential component of the speed of the fluid at each point is compensated by the internal depressions and the released flux is very slightly divergent. This does not apply to a propeller turning in a finite space (a mixer basin, for example) and where a centrifugal component of speed is produced. It follows that the fluid streams do not remain parallel to the axis of rotation and that, in particular, the central streams leave the propeller on a radius higher than that of entry and in a very divergent direction in relation to the axis of rotation. The result is a zone of low efficiency flux adjacent the propeller, in the vicinity of the roots of the blades.